In recent years, attention has been focused on solar energy as an energy source because of environmental problems and so forth.
Methods of converting light or heat of solar energy into electrical energy as usable energy have become practically available.
Among these methods, a method of converting sunlight into electrical energy, for example, is a typical example, and a photoelectric conversion element is utilized in this method.
The photoelectric conversion element in which an inorganic material such as single crystalline silicon, polycrystalline silicon, amorphous silicon, and cadmium telluride, indium copper selenide or the like is employed is widely used as a photoelectric conversion element, and has been utilized for a so-called solar cell.
A solar cell employing a photoelectric conversion element in which an inorganic material is used has had problems, for example, a high purity product obtained via a high purification process is needed for silicon used as a material, and the manufacturing processes is complicated and contains a lot of processes since a multilayered pn junction structure is employed, whereby the manufacturing cost is high.
On the other hand, studies of a photoelectric conversion element in which an organic material is used as a simple element have been also in progress.
As described in C. W. Jang: Applied Physics Letters, 48, 183 (1986), for example, reported is a p-n junction type organic photoelectric conversion element in which a perylenetetracarboxylic acid derivative as an n-type organic dye and a copper phthalocyanine as a p-type organic dye are joined.
In order to improve a short exciton diffusion length and thin thickness of a space-charge layer taken into account as a drawback, results thereof are being made by largely increasing the area of a p-n junction region where organic thin films are simply layered to sufficiently secure the number of organic dyes which contribute the charge separation.
Further, as described in G. Yu, J. Gao, J. C. Humelen, F. Wudl and A. J. Heeger: Science, 270, 1789 (1996), disclosed is a technique by which the p-n junction region is largely increased by mixing a n-type electron-conductive organic material and a p-type hole-conductive polymer in the film to conduct charge separation in the entire film. Proposed was a photoelectric conversion element in which Heeger et al. made a conjugated polymer to be a p-type conductive polymer in 1995 to mix fullerene as an electron-conductive material.
Such a photoelectric conversion element gradually exhibits an improved property thereof, but stable behavior with high conversion efficiency has not been obtained yet.
However, in 1991, Gratzel succeeded in preparation of a photoelectric conversion element having a high conversion efficiency in stable operation by preparing porous titanium oxide, and sufficiently securing the charge separation area (the number of molecules contributed for charge separation) in detailed experiments having been enormously compiled (for example, refer to Non-Patent Document 1).
In the case of this photoelectric conversion element, repeated are cycles in which a dye adsorbed onto the surface of porous titanium oxide is optically excited to form a dye cation via electron-injection from the dye to the titanium oxide, and the dye receives electrons from the counter electrode via a charge transport layer. An electrolytic solution in which an electrolyte containing iodine is dissolved in an organic solvent is used as a charge transport layer.
In combination with the stability of the titanium oxide, this photoelectrical conversion element exhibits excellent reproducibility, the R&D base thereof has largely expanded, and this photoelectric conversion element receives notable expectation and attention while it is also called a dye-sensitized solar cell. This technique has an advantage to effectively convert sunlight having a large amount of visible light components into electricity since it is not necessary to purify an inexpensive semiconductor material such as titanium oxide into a high purity grade and thus an inexpensive semiconductor material can be used, and the usable light expands to a broad range of visible light.
However, these conventional dye-sensitized solar cells have had problems that the obtained voltage is not fully sufficient as the result of an inverse current which occurs independently of the light irradiation.
As a technique to overcome the problem of such a lower obtained voltage, proposed has been a technique to incorporate an amino-pyridine compound in the electrolyte solution of a photoelectric conversion element having a semiconductor electrode, a counter electrode and an electrolyte solution to obtain a high open circuit voltage (refer to Patent Document 1).
Further, in a dye-sensitized photoelectric conversion device, techniques to remove a dye excessively adsorbed on a semiconductor electrode have been known, for example, a technique to conduct a treatment on the surface of a conductor layer with an amine compound such as pyridine, 4-t-butylpyridine and polyvinylpyridine after a dye has been adsorbed (refer to Patent Document 2), and a photoelectric conversion element having a charge transport layer containing 4-t-butylpyridine (refer to Patent Document 3).
However, the problem has been that the photoelectric conversion efficiency has not been fully enough even in such photoelectric conversion elements.    Patent Document 1 Japanese Patent Application Publication Open to Public Inspection (hereafter referred to as JP-A) No. 2004-47229    Patent Document 2 JP-A No. 2009-81141    Patent Document 3 JP-A No. 2010-267612    Non-Patent Document 1 B. O'Regan and M. Grätzel: Nature, 353, 737 (1991)